Laser Doppler velocimetry (LDV), which is also referred to by laser Doppler anemometry (LDA) represents a measurement technique for determining a flow characteristics of a movable object using the Doppler effect. In particular, Laser Doppler velocimetry is usable in conjunction with medical diagnosis applications, in order to, for example, determine a flow characteristic of micro vascular blood or tissue perfusion characteristics of a person. In the medical area, LDV is also referred to as laser Doppler flowmetry (LDF). Alternatively, the technique may be used in the area of material machining, in order to determine a flow characteristic of a material particle stream. In the following, reference is made to the medical area without loss of generality.
One LDF based technique for determining the flow characteristic of blood of a person will be explained with reference to FIG. 1. Coherent laser light 102 emitted by a laser unit is incident on a skin portion 104 of a skin 106 of the person. The light 102 comprises a frequency ω0, as illustrated in a diagram 108 with an ordinate of the diagram 108 representing a frequency (measured in Hertz) and an abscissa of the diagram 108 representing an intensity of the emitted light 102 (measured in arbitrary units). The light 102 penetrates into surface layers of the skin 106 beneath the skin portion 104, and is amongst others scattered at blood cells 108a-e moving in the skin 106. Multiple scattering events of the light 102 at different blood cells 108a-c, e as well as a single scattering event of the light 102 at one blood cell 108d are illustrated in FIG. 1 for illustration purposes. Respective back-scattered light 110a-c propagates subsequent to the scattering event(s) to a detector 112. Reflection of the light 102 at the skin portion 104 on which the light 102 is incident may also cause reflected light 110d to propagate to the detector 112. The detector 112 detects all incoming light 110a-d with the detected light comprising a frequency distribution centered around the frequency ω0 of the light 102, as illustrated in a diagram 114. An ordinate of the diagram 114 represents a frequency (measured in Hertz) and an abscissa of the diagram 114 represents an intensity of the detected light (measured in arbitrary units).
Ideally, in accordance with the Doppler effect, each of the back-scattered light 110a-c comprises a frequency ω0+Δω with Δω denoting a frequency shift compared to the initial frequency ω0 of the light 102. The frequency shift Δω is determined by a vectored velocity of the respective blood cell(s) 108a-e and the directional change of the incident and scattered light. In the one-dimensional case, a reduction of the distance between a moving blood cell and the laser unit leads to a positive signed frequency shift Δω, while an increase of the distance between a moving blood cell and the light source leads to a negative signed frequency shift Δω. The reflected light 110d ideally comprises the frequency ω0 of the light 102.
The frequency distribution of the detected light illustrated in the diagram 114 is caused by several effects. Further, a clear frequency shift Δω may not be observed owing to random velocity values of the blood cells. The blood cells 108a-e may move in all directions and comprising different velocity values, thereby leading to different values of the frequency shifts Δω of the back-scattered light 110a-c. 
The flow characteristic of the blood is then determined based on the obtained frequency spectrum.
Although back-scattered light is predominately illustrated in FIG. 1, the detected light may comprise a high fraction of the reflected light. Thus, the above described measurement technique may comprise low depth sensitivity, and, when considering a small Doppler frequency shift, an accuracy of the determination of the flow characteristic of the blood may be low.
A further option for the determination of the flow characteristic of the blood based on the Doppler effect additionally employs self-mixing interferometry (SMI). A respective SMI-LDF based measurement principle will be explained with reference to FIG. 2. A laser unit 220 emits light 222 towards a skin portion 224 of a skin of a person. A distance between a front side of the laser unit 220 and the skin portion 224 is indicated in FIG. 2 by s. Light 226 reflected or scattered back from the skin portion 224 enters again the laser unit 220. Mixed light 228 is generated in the laser unit 220 in that light in the laser unit which is to be emitted (and thus corresponds to the emitted light 222) and the detected back-scattered and reflected light 226 mix in the laser unit 220. The mixed light 228 is outputted by the laser unit 220, and is detected by a photodiode 230 arranged externally of the laser unit 220. A power spectrum obtained by the photodiode 230 is analyzed using a so called three-mirror Fabry-Perot cavity model with the front surface, now referenced by 232, and a rear surface 234 of the laser unit 220 and the surface 236 of skin portion 224 representing the respective mirrors. A result of the analysis provides information about the flow velocity of the blood in the skin portion 224.
As stated above, the determination of the flow characteristic of the blood based on SMI may also suffer from a poor accuracy.
WO 2009/027896 describes a SMI based method and apparatus for measuring skin properties of a person, for example a dehydration level of the skin. The apparatus comprises a laser sensor configured for transmitting laser light towards the skin portion to be investigated and for receiving laser light reflected from the skin portion. The laser sensor comprises a photodiode configured for measuring power fluctuations of the laser light of the laser sensor, in order to determine a recoil velocity of the skin portion based on a change of the power fluctuation of the laser light over time. Due to the self mixing effect the back-scattered light gives rise to power fluctuation of the laser. A polarizer of the apparatus is arranged between the laser sensor and the skin portion, in order to suppress fractions of the reflected light which comprises a different polarization compared to the emitted light. The skin property is determined based on the power fluctuations.